Complex analog integrated circuits typically include circuitry to provide access to some internal analog signals for test purposes. To access internal analog signals, the internal analog signals routed as inputs to a multiplexer that is used to select one of the signals as the input to an analog output buffer. The buffer functions to buffer the external load, such as the test equipment, from the integrated circuit that is not designed to drive an external load.
The prior art has many forms of buffer circuits. One widely used prior art buffer circuit includes a differential source coupled pair of transistors with an output source follower. This circuit provides high gain and large bandwidth but has the shortcoming of limited input voltage swing, particularly in the positive direction. The source follower transistor prevents the input voltage from reaching the positive voltage rail due to the threshold and ON voltage of the source follower transistor. A further shortcoming of this circuit is the necessity to continuously sink a large current required for discharging an external capacitive load. For a rapid discharge, a large current is required. The sink current must also be provided by the source follower transistor when charging the external load. Thus, the source follower consumes large amounts of power.
A circuit that appears to overcome shortcomings of the above-mentioned circuit is disclosed in IEEE Journal of Solid State Circuits, Vol. 26, No. 6, June 1991, pages 859-867, the disclosure of which is hereby incorporated by reference. The circuit is depicted in FIG. 3C of the article and is an operational transconductance amplifier, or basic g.sub.m cell. The article teaches using a composite differential pair transistor with current mirrors to provide the output currents. The circuit has linear characteristics voltage-to-current over a large differential input voltage range. The input voltage signal levels are maintained as large as possible to obviate signal-to-noise ratio problems. The linear output versus input characteristics are obtained without feedback in the circuit. The p-channel devices serve as voltage followers buffering the input voltage across the resistor while the four constant current sources force any change in the resistor current to be directly reflected to the drain currents of transistors M2 and M2' which is mirrored to the output by transistors M3 and M3'.
Linear voltage transfer characteristics are typically obtained by employing linear feedback elements around a very high gain voltage amplifier. For use as a voltage buffer, this circuit has the shortcoming of low voltage-to-current gain which results in limited bandwidth. Wide bandwidth is important because the buffer must have a bandwidth at least as great as the bandwidth of the intermediate signals to prevent attenuation of frequencies in the internal analog signals being presented at the buffer output. Ideally, the gain of the buffer should be unity across the entire frequency range of interest so no attenuation of the intermediate signal occurs. This would assure that the intermediate signal is not distorted by the buffer.
It would be desirable to have a highly linear voltage buffer that would provide a wide bandwidth and consume little power. Such a buffer would have a large transconductance such that it would only require a small differential voltage input to result in a relatively large change in output current, thereby providing low distortion.